More automation for release : changelog

[brunob]

As mentioned in https://github.com/leaflet-extras/leaflet-providers/pull/282#issuecomment-477912150 :

maybe we could use a tool like https://github.com/conventional-changelog/standard-version for changelog+release ?

standard-version use conventional-changelog to generate changelog files, i've tested it with conventional-changelog -i test.md -s -r 0 using no preset since we are not following any of the commit conventions available. The generated changelog looks good to me, see this extract :

---

1.6.0 (2019-03-28)

• Bumped version to 1.6.0 (2f76cf3)
• changelog for 1.6.0 (0e9511c)
• Disable HERE in preview (dc03ba3)
• RIP OpenStreetMap Black and white (6f7b2a5), closes #316
• Serve tiles from wmflbs.org over https (#311) (2ce6a14), closes #311
• Tile attribution improvements (#309) (29fcb51), closes #309
• update openmapsurfer tile url (#310) (c3a6f1b), closes #310
• Use explicit name for release patch too (a6c0ac2)

1.5.0 (2018-11-06)

• Add changelog for 1.5.0 (1a82827)
• Avoid a 301 redirect for Stamen watercolors (#299) (8d43952), closes #299
• Fix #301 (36b6edd), closes #301
• Fix #302 (#303) (76984ae), closes #302 #303
• Fix #304 (65bb0b7), closes #304
• HERE maps update (#307) (8087ff8), closes #307
• Reduce the number of HERE requests, cleanup (d44ff56)
• Remove OpenInfraMap fixes #300 (d7aad06), closes #300
• update carto url & attribution (cartodb => carto) (#305) (c58760d), closes #305
• Update devdependencies (54b5b35)
• Updating OneMapSG Attribution with OneMapSG Logo (#297) (4fb7114), closes #297 /github.com/leaflet-extras/leaflet-providers/pull/295#issuecomment-416699821
• v1.5.0 (c243c92)

1.4.0 (2018-08-25)

• Added here baselayers (a8f4039)
• Added OneMap (Singapore) (#295) (069bf6e), closes #295
• fix typo (a504869)
• Reduce the amount of LOC for OneMapSG after merging #295 (9732ecb), closes #295
• Update changelog prior to releasing 1.4.0 (be7d043)
• Use mocha-chrome instead of phantomjs (555c8d4)
• v1.4.0 (556508d)